Conventionally, JIS SKD11 has been in heavy usage for a cold die steel, but some attempts have been made to modify the SKD11 in order to improve machinability, toughness and hardness after secondary hardening. For instance, there proposed are (1) a cold die steel referred to 10% Cr SKD (see JP-A-11-279704) which has improved machinability and toughness, in which amounts of added carbon and chromium are adjusted so as to reduce non-solute carbides while the matrix composition are maintained to be that of SKD11 as much as possible, and (2) a cold die steel referred to 8% Cr SKD (see JP-A-01-011945) which has improved secondary hardening capability, in which an amount of non-solute carbides are reduced and an amount of molybdenum is increased while the matrix composition are maintained to be that of SKD11 as much as possible.
The above described techniques are effective to improve characteristics required for a cold die steel. However, these techniques have problems to cause a large dimensional change in a tempering step. Specifically, a great dilatation in a secondary hardening region in tempering leads to increase working steps after heat treatment.
The dilatational change of dimension in tempering is caused by a release of a residual stress formed in a previous quenching step (by decomposition of retained austenite), and is promoted by precipitation of temper carbides formed by molybdenum and the like which are conventionally added expecting for secondary hardening. In addition, if the retained austenite is restrained by non-solute primary carbides which have been formed in casting and originally existed, the decomposition thereof in the tempering step is suppressed. However, it is preferable to reduce the primary carbides because they are a factor of degrading machinability, and so the reduction thereof promotes the decomposition of the retained austenite and the dimensional change.